Temperature effects on photoluminescence of YAG:Ce^(3+) phosphor and performance in white light-emitting diodes

The well crystalline YAG：Ce^3＋ phosphor was synthesized by sold-state method, and the temperature dependence of excitation and emission spectra of YAG：Ce^3＋ phosphor were investigated in the temperature range from room temperature to 573 K. With temperature increasing, it was noted that the emission intensity of as-repared phosphors decreased considerably more rapidly when pumped by 460 nm than by 340 nm. The temperature-intensity curves under different excitation wavelengths were obtained using an Arrhenius function, and the corresponding activation energies were also obtained respectively. Thus, the experimental phenomenon was discussed in terms of nonradiative decay rate. The effects of as-prepared phosphors on the performance of the white LED with changing temperature were also studied.

Effect of Precursor Molar Ratio of [S^2-]/[Zn^2＋] on Particle Size and Photoluminescence of ZnS：Mn^2＋ Nanocrystals

We investigate the influence of precursor molar ratio of [S^2-]/[Zn^2＋] on particle size and photoluminescence （PL） of ZnS：Mn^2＋ nanocrystMs. By changing the [S^2-]/[Zn^2＋] ratio from 0.6 （Zn-rich） to 2.0 （S-rich）, the particle size increases from nearly 2. 7nm to about 4.Ohm. The increase in the ratio of [S^2-]/[Zn^2＋] cadses a decrease of PL emission intensity of ZnS host while a distinct increase of Mn^2＋ emission. The maximum intensity for the luminescence of Mn^2＋ emission is observed at the ratio of [S^2-]/[Zn^2＋] ≈ 1.5. The possible mechanism for the results is discussed by filling of S^2- vacancies and the increase of Mn^2＋ ions incorporated into ZnS lattices.

White Emitting ZnS Nanocrystals： Synthesis and Spectrum Characterization

Spherical organic-bonded ZnS nanocrystals with 4.0 4-0.2 nm in diameter are synthesized by a liquid-solid-solution method. The photoluminescence spectrum of sample （[S^2-]/[Zn^2＋] = 1.0） shows a strong white emission with a peak at 490nm and - 170 nm full widths at half maximum. By Gauss fitting, the white emission is attributed to the overlap of a blue emission and a green-yellow emission, originating from electronic transitions from internal S^2- vacancies level to valence band and to the internal Zn^2＋ vacancy level, respectively. After sealingZnS nanocrystals onto InGaN chips, the device shows CIE coordinates of （0.29,0.30）, which indicates their potential applications for white light emitting diodes.

Influence of reaction temperature on the formation process of ZnO quantum dots and the optical properties

ZnO quantum dots (QDs)with the sizes of 3.0-5.6 nm are synthesized by solution-phase method at different temperatures. We find that temperature has great influence on the size of ZnO QDs. The growth process is the most sensitive to temperature, and the process is well explained by Lifshitz-Slyozov-Wagner (LSW) model. By photoluminescence (PL) spectra of the quantum dots at different temperatures and reactive time, we come to a conclusion that ultraviolet emission is mainly due to surface defects, and the origination of green visible emission comes from the transition between the bottom of the conduction band and the level of Oi.

Influence of Si^(4+) substitution on the temperature-dependent characteristics of Y_3Al_5O_(12):Ce

A series of aAl5O12：Ce （YAG：Ce） phosphors doped with different Si4＋ concentrations is prepared by solid-state reaction. The temperature dependent characteristics of luminescent spectrum and decay time of Ce3＋ are investigated. With Si4＋ doped, the emission spectrum shows a blue shift clue to a decrease of the splitting of 5d levels of Ce3＋ ion. The thermal stability is greatly improved by adding Si4＋ because the activation energy AE increases from 0.1836 eV to 0.2401 eV. The study of the decay times against temperature for various doping concentrations of Si4＋ shows that the calculated nonradiative decay rate is affected by Si4＋ substitution. The results are explained by the configurational coordinate diagram.

Structural investigation and luminescent properties of BaZr(BO_3)_2:Eu^(3+) phosphors containing Si

Si4+-doped BaZr(BO3)2:Eu3+ phosphors are prepared by a conventional solid-state reaction.The influence of Si4+ addition on the charge transfer state of Eu3+-O2- and photoluminescence(PL) properties of BaZr(BO3)2:Eu3+ are discussed.Room temperature PL spectra indicated that efficient emission is obtained by Si doping.Increased values for the peak-peak ratio(PPR) of BaZr(BO3)2:Eu3+ at higher Si doping concentrations implied that the Eu3+ ion is located in a more asymmetric environment in BaZr0.8Si0.2(BO3)2:Eu3+ than in the undoped samples.The Judd-Ofelt parameters Ωλ(λ=2,4) were calculated from the PL data,giving results that were consistent with those from the PPR.The maximum radiative quantum efficiency was achieved at a Si doping concentration of 20 mol%.

Controlled growth of ZnO nanorods by polymer template and their photoluminescence properties

A large amount of one-dimensional ZnO nanorods with diameters in 15―50 nm aligned in radial cluster were successfully synthesized by polar polymer polyvinyl alcohol (PVA) as soft-template. The growth of ZnO nanorods was controlled by changing annealing temperature. The evolution of the morphology and microstructure was investigated by scanning electron microscope, transmission electron micro- scope and X-ray diffraction. It is shown that ZnO nanorods tend to be uniform and the crystallization is gradually improved with the temperature increasing from 400℃ to 700℃. The photoluminescence spectra of products show a strong ultra violet emission and relatively weak defect emissions. The sharp strong emission peak at 354 nm owing to the inter-band transition indicates the extraordinary photoluminescence property of ZnO nanorods.

MBenes:Two-dimensional transition-metal borides with ordered metal vacancies

2D MBenes have been theoretically predicted to possess unique electronic structures and physicochemical properties,and thus shown great promise in various applications.However,the synthesis of individual single-layer MBene remains a grand challenge due to its orthorhombic structure of MAB phases.Recently,scientists from Link?ping University have fabricated 2D monolayer Mo4/3B2-xTzwith ordered metal vacancies.Their results demonstrated the feasibility of top-down approach by chemical exfoliation of laminated compounds and provided the principle for further preparation of a wealth of MBenes.

Effect of precursor molar ratio of [S^(2-)]/[Zn^(2+)] on the lifetime of ZnS nanocrystals

Undoped ZnS nanocrystals(NCs) with different precursor molar ratios of [S2-]/[Zn2+] are prepared by the chemical precipitation method.The structural and optical properties of the samples are characterized by the X-ray diffraction(XRD) spectra,photoluminescence(PL) spectra and PL decay spectra.The XRD analysis shows that the crystal quality of ZnS NCs becomes better and the grain size is larger at higher [S2-]/[Zn2+] ratios.The PL peaked at 430 nm decreases with the [S2-]/[Zn2+] ratio increasing,which is ascribed to the structure defects of NCs.A multi-exponential decay time curve with hundreds of picoseconds,several nanoseconds and tens of nanoseconds is obtained,which also shows a distinct and regular change with [S2-]/[Zn2+] ratio.It is indicated that the PL and emission decay properties of ZnS NCs mainly depend on the change of the defects number from different particle sizes.

Fluorescent emission characteristics of polycrystalline diamond film prepared by direct current jet CVD

The free-standing diamond films are deposited on molybdenum substrate by direct current jet chemical vapor deposition (DCJCVD). X-ray diffraction,Raman spectroscopy and cathodoluminescence (CL) measurement are used to investigate the films structure and defects related to electron transition properties of the diamond films. The X-ray diffraction spectrum reveals that the diamond films have the polycrystalline cubic structure with diffraction peaks at 43.88o and 75.24o. A sharp peak at 1331.8 cm-1 and a broad band at about 1250-1550 cm-1 from Raman spectrum are attributed to diamond phase and sp2-type carbons,respectively. Two emission peaks at 440 nm and 530 nm,associated with dislocation defects and nitrogen and vacancy complexes respectively,are observed in cathodoluminescence spectrum. In addition,in order to understand both emission processes,a simple energy level scheme is suggested.